Do Capitals In Email Addresses Make Any Difference?

          May 29, 2019

Mostly you see email addresses that have no capital letters, but you may have seen email addresses that were all caps or mixed caps and small letters. You may have wondered whether you can use caps yourself when giving out your own email address, perhaps to emphasize some part of it or indicate the start of individual words in a multi-word address. In more technical terms, you wondered whether email addresses are "case-sensitive".

The answer is caps shouldn't make any difference, but given the plague of IT incompetence there may be some bad programming along the way that results in caps causing problems and emails not reaching their destinations.

Email addresses consist of a username, which is that of the email server computer account, and a domain name, which gives the email server computer's address on the Internet. For example, in the email address dr.duane.thresher@apscitu.com, dr.duane.thresher is the username and apscitu.com is the domain name. (As part of Apscitu Mail, I administer the apscitu.com email server computer myself.)

Capitals can make a difference in the username. Email server computers that use Linux as the operating system are case-sensitive. So for example, Dr.Duane.Thresher would be a different user than dr.duane.thresher. However, accounts on properly administered Linux computers are always created with no-caps usernames. Email server computers that use the more simplistic Microsoft Windows are not case-sensitive in the first place.

More importantly, on a properly administered email server computer, when an incoming email address is checked, by comparison, for whether it is to go to an account there, the incoming email address username is, via programming, automatically made into no-caps before comparison.

Domain names are defined as no-caps in the Internet standards. Otherwise, hackers would go crazy with getting domain names for their scam email addresses that differ just by capitalization from legitimate domain names and email addresses, so that they could email people and easily fool them into emailing back important private information ("phishing"); for example, SUPPORT@GOOGLE.COM instead of support@google.com ("We need your password for account maintenance. Otherwise your account will be deleted.").

The domain name is converted to an IP address, for routing on the Internet, by a DNS (Domain Name System) server computer on the Internet. This conversion is, for the reason given, programmed not to be case-sensitive.

An email server computer often hosts more than one domain name and uses the domain name in the email address to determine which account to send the email to. A properly administered email server computer is usually programmed to not be case-sensitive.

Thus the same no-caps programming used for usernames is applied to domain names.

This is very basic programming, i.e. basic IT competence, but given the plague of IT incompetence, sometimes caps in email addresses cause problems. I myself have had this problem with the U.S. Postal Service (USPS) on their usps.com website; see The U.S. Mail SHOULD Be Worried About Email Competition.

In general it's safest to just use no caps in email addresses, but for my own email addresses on my Apscitu Mail email server computers, capitals and small letters can be safely mixed.